Is Taking Tylenol PM Above Prescribed Dosage Harmful?

I have tylenol pm that i use every once in a while to help me sleep. I am going to work out for 2 hours then come back and try to take a nap. it is very critical that i am able to sleep during this time so i can catch up on my sleep. It says on the label not to take more than 2 caplets every 24 hours. but i was wondering if anything bad would happen if i just took 2 this morning after taking 2 about 10 hours ago? Also i am 16 years old, 6 feet tall and about 170 lbs. my medical history is quite normal, no allergies or anything.